Dye-sensitized solar (DSCs) cells are low-cost photovoltaic devices that can provide high solar energy conversion efficiencies. Conventional DSCs consist of a wide band gap semiconductor oxide (e.g., TiO2, SnO2 or ZnO), Ru complex photosensitizing dye molecules loaded onto the semiconductor oxide, electrolytes containing I−/I3− redox couples in organic solvents and Pt-coated counter electrode. The role of the electrolyte is to transfer electrons to the photosensitizing dye molecules and also to transport the positive charges to the counter electrode. High conversion efficiency, long-term stability and facile and low-cost fabrication are essential for commercialization of DSCs. Unfortunately, iodide-containing electrolytes commonly used in DSCs are highly corrosive and volatile, resulting in reduced cell performance control, reduced long-term durability and incompatibility with some metallic components, such as Ag, in the presence of water and oxygen.
Research focused on replacing conventional fluid electrolytes for DSCs with p-type semiconductors and solid-state hole-transport materials (HTMs) has been conducted. These alternative electrolyte materials are mostly organic molecules, such as sprio-OMeTAD (2,2′,7,7′-tetranis(N,N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene) and p-type conducting polymers, such as polypyrole, polydiacetylene, poly(3-octylthiophene), poly(3,4-ethylenedioxythiophene) (PEDOT) and 2,2′-bis(3,4-ethylenedioxythiophene) (bis-EDOT). As an organic HTM, spiro-OMeTAD has shown an efficiency of η=5.1%. As a conducting polymer HTM, (bis-EDOT) with Z907 dye provided an efficiency of η=6.1%. A DSC with an iodide-free, organic redox electrolyte of disulfide/thiolate molecules has shown an efficiency of η=6.4%.
Despite many advantages, inorganic HTMs are uncommon. Examples are CuI, CuSCN and NiO. The efficiency of CuI-based DSCs has been reported to be about 6%. However, formation of Cu2O/CuO and the release of iodine on standing rapidly degraded CuI/TiO2 interfaces. Cells of CuSCN showed an efficiency of about 2%. P-type NiO particles exhibited very poor performance as a hole transport layer.